The present invention relates to a storage capacitor in a semiconductor memory device and a method of forming the same, and more particularly to a storage capacitor having a refractory metal storage electrode for semiconductor memory devices such as dynamic random access memory device.
In recent years, improvement in high integration of the dynamic random access memory has been required, whilst this makes it difficult to secure a sufficiently large capacity of each memory cell in the dynamic random access memory due to the requirement for reduction in occupied area of each storage capacitor in the cell. In order to secure a sufficiently large capacity of the memory cell, it is effective to reduce the thickness of a capacitive insulation film of the storage capacitor and also to increase a surface area of interfaces of the storage electrode and the capacitive insulation film.
In order to reduce the thickness of the capacitive insulation film, it is effective to use a capacitive insulation film of an insulation material having a higher dielectric constant than that of a silicon nitride film.
One of the storage capacitor having a capacitive insulation film made of an insulation material having a high dielectric constant is disclosed in the Japanese laid-open patent publication No. 3-136361. This conventional storage capacitor will be described with reference to FIG. 1. A capacitive insulation film 1 is made of tantalum oxide Ta.sub.2 O.sub.5 which has a high dielectric constant. A storage electrode 2 is made of tungsten because tungsten has a sufficient resistivity to oxidation which is carried out to form the capacitive insulation film 1. Namely, a tungsten storage electrode 2 is preferable to prevent the storage electrode 2 from oxidation when the capacitive insulation film 1 is formed. The prevention of the storage capacitor from oxidation results in prevention of the drop of the capacity of the storage capacitor. If the capacitive insulation film is made of an insulation material having a high dielectric constant in order to ensure a sufficiently high capacity of the storage capacitor, then any refractory metal such as tungsten is often used.
On the other hand, a method of increasing an area of an interface between the capacitive insulation film and the storage electrode is disclosed in the Japanese laid-open patent publication No. 3-69162. This method will be described with reference to FIG. 2. A storage electrode 4 is made of polysilicon. Further, polysilicon side walls 5 are formed at side walls of the ends of the polysilicon storage electrode. The polysilicon side walls serve as parts of the storage electrode in cooperation with the polysilicon storage electrode 4. The polysilicon side walls 5 substantially increase the area of the interface of the storage electrode and the capacitive insulation film. A distance between adjacent two storage electrodes is defined by a resolution limitation of an exposure system although the distance is preferred as small as possible. If the polysilicon side walls are provided at the side walls of the storage electrode, then the distance between the adjacent two storage electrodes is made small by the lateral size of the polysilicon side walls. This results in an increase in area of the interface between the polysilicon storage electrode united with the polysilicon side walls 5 and the capacitive insulation film. Actually, however, it is difficult to precisely form the polysilicon side walls 5 only on the side walls of the polysilicon storage electrode 4.
A method of easily forming polysilicon side walls on side walls of the polysilicon storage electrode 4 is disclosed in the Japanese laid-open patent publication No. 3-165552. This method will be described with reference to FIG. 3. Polysilicon storage electrodes 7 have been formed for subsequent formation of silicon oxide films 8 only on the polysilicon storage electrodes 7 before a polysilicon film is entirely deposited and then subjected to etch back process, wherein the silicon oxide film 8 serves as an etching stopper to the etch back. As a result of the etch back, the polysilicon film remain only on the side walls of the polysilicon storage electrodes 7 whereby polysilicon side walls 9 are precisely formed on the side walls of the polysilicon storage electrode 7.
In order to obtain a further increase in capacity of the storage capacitor, it was proposed to use the above first conventional technique in combination with the above second or third conventional techniques. Namely, in place of polysilicon, tungsten is used to form a tungsten storage electrode with tungsten side wall films in order to increase the capacity of the storage capacitor. In this case, once a tungsten film is entirely deposited on a silicon oxide inter-layer insulator before the tungsten film is defined into the storage electrode. The adhesion strength between the tungsten film and the silicon oxide film is weak, for which reason the tungsten film is likely to be peeled from the silicon oxide film while tungsten particles remain on the silicon oxide film, resulting in a low yield of the memory device.
If the silicon oxide film serving as the etching stopper is formed over the tungsten storage electrode, then a chemical reaction may be caused on an interface between the silicon oxide film and the tungsten storage electrode. If silicon in the silicon oxide film moves into the tungsten storage electrode by the chemical reaction, then a tantalum oxide capacitive insulation film is deteriorated in electric property. If such silicon oxide film etching stopper was once formed, it is required to remove this etching stopper from the top surface of the tungsten storage electrode by etching process. If the silicon oxide etching stopper film is etched, then the silicon oxide inter-layer insulator on which the tungsten storage electrodes are selectively formed is also subjected to this etching. As a result, when the silicon oxide etching stopper is etched, the silicon oxide inter-layer insulator is also etched to form recessed portions therein. The formation of the recessed portion in the silicon oxide inter-layer insulator results in a deterioration in electric property of the capacitive insulation film.
In the above circumstances, it had been required to develop a novel storage capacitor structure and a novel method of forming the same to allow a facilitated formation of the refractory metal side wall films on side walls of the refractory metal storage electrode without any problem in peeling a refractory metal film from an inter-layer insulator.